1. Field of the Invention
This invention relates to photon tunneling microscopy and is particularly directed to flexible transducers including their method of fabrication and use.
2. Description of the Prior Art
Photon tunneling microscopy for measuring and visualizing submicron surface topographic features is known. U.S. Pat. No. 4,681,451, issued Jul. 21, 1987 to John M. Guerra, et al. discloses an optical imaging method and apparatus for practicing photon tunneling microscopy. Here, the proximity of a glass surface to a sample surface is determined by frustration of total internal reflection of light energy at the glass/sample interface to develop a patterned area of gray tones. The densities of the gray toned pattern are calibrated such that levels of density correspond to surface proximity. A facsimile of the gray scale image is displayed to indicate variations in sample surface proximity. The pattern of reflected light preferably is magnified, and the magnified image is recorded by a television camera. In a preferred embodiment, the output of the television camera is fed to an oscilloscope adapted to display a three axis image in which one of the three axes corresponds to variation in gray scale density. The output of the television camera may also be fed through a colorizer to assign different colors to different gray scale densities, and a colored facsimile of the pattern image may be presented on a CRT screen.
The method and apparatus of the '451 patent find application in fields in which there is a need for accurate measurement and/or observation of extremely small spacing gaps between surfaces of physical components or of surface features. For example, increases in information bit density of recording media has necessitated correspondingly smaller head gaps to assure accurate transducing of information stored on such recording media. By substitution of a glass body for the conventional magnetic head, the method and apparatus of the '451 patent facilitate observation and quantification of the spacing of a magnetic head relative to the recording media, which may be a magnetic storage tape or disk. Spacings of one microinch may be studied with this technique.
The '451 apparatus features a rigid optically transmissive body, such as a glass block, having thereon the aforementioned glass surface. The glass surface, which can simulate the surface configuration of the magnetic recording head, is substantially flat. Other configurations may be used, and materials other than glass may be used, but in each instance, the optically transmissive body, including the surface proximate the sample surface, is rigid.
When two such surfaces are brought close together, there is a probability that photons tunnel across the gap between the glass surface and the sample surface. The distance of penetration with no gap is approximately 0.75X the illuminating wavelength. Because of this phenomenon, this form of microscopy is known as photon tunneling microscopy. Here, the glass block is known as a "transducer" because the block changes height variations or separations into light modulation. It will be apparent that as the gap enlarges, i.e., the separation between the glass surface and the subject surface increases, fewer photons tunnel; and as the gap increases, more photons tunnel. Thus, variations in the depth of various points in the sample surface produce different degrees of frustration of internal reflection which, in turn, produce observable and measurable imagery.
Photon tunneling microscopy is also described in an article entitled "Photon Tunneling Microscopy" by John M. Guerra, which appeared in Applied Optics, Sep. 10, 1990, pages 3741-3752. Described therein is a photon tunneling microscopy apparatus including an optical microscope equipped with vertical illumination, a phototube, and a reflected light oil immersion objective of numerical aperture of greater than 1.0. Here, a microscope cover glass transducer is oil immersed and acts as part of the objective, the distal surface of the transducer being substantially coplanar with the object plane for total internal reflection. A photometric vidicon camera mounted on the phototube converts the gray scale tunneling image into a video signal that is restored by a three-axis oscilloscope as a real time three-dimensional image of the sample microtopography. Additional instrumentation may include an image processor, video recording devices, and a relay video camera that images the three-axis oscilloscope display onto a larger monitor.
Illumination is vertical, introduced to the objective via a beam-splitter which receives light from an illuminator fixed to the microscope tube. The objective is substantially focused on the distal surface, i.e., the glass-air interface, of the glass transducer. The transducer, which is a standard microscope cover glass measuring 39.times.33.times.0.2 mm and of rigid construction, is oil immersed and illumination from N.A. 1 to N.A. 1.25 is reflected totally.
In the method disclosed in the Applied Optics article, the rigid transducer is placed onto the sample surface such that the highest peaks in the sample topography contact it. The remainder of the topography forms gaps of varying thicknesses with the transducer. Since the probability of photon tunneling decreases exponentially as the gap increases, the light returning to the microscope is modulated by the topography through photon tunneling and partially frustrated total internal reflection. The result of the partially frustrated internal reflection is a light pattern exiting from the transducer. Peaks appear as dark patches in the subject surface where there is maximum frustration of internal reflection. Increasingly brighter areas arising with a diminution of frustration correspond to increases in internal reflection where the subject surface diverges from the transducer surface. Any sample surface removed from the transducer by a distance greater than the range of the photon microscope, typically about 0.3 microns in the green region of the spectrum, will appear as a white area.
The modulated light forms a grayscale image of the sample in which the grayscale represents height. The tunneling image is viewable directly by eye or with a CCD, or vidicon, can be converted to a three-dimensional representation by an x-y display in which scenic video brightness containing height information is displayed as amplitude where it is perceived as height.
The transducer in the photon tunneling microscope disclosed in the Applied Optics article is a rigid plano-parallel glass body. The distal surface of the transducer, or glass body, is in the object plane of an oil immersion objective, preferably has a numerical aperture greater than unity, and is adapted to be brought into dry optical contact with the sample surface.
While the method and apparatus disclosed in the '451 patent and the Applied Optics article have proven successful in imaging and measuring subnanometer surface topography in real time, the combination of a rigid transducer and subwavelength proximity of the transducer to the sample leads to several problems.
First, there is a problem related to surface cleanliness. The transducer and the sample surface must be free of particulate matter in order to effect the close proximity required for tunneling. Dirty surfaces require cleaning of the sample, which, in turn, can lead to sample damage. Moreover, cleaning without inflicting damage to the sample requires considerable operator skill.
Second, the transducer in such close proximity to a rigid sample surface may itself be damaged or may cause damage to the sample.
Third, a rigid, plano-parallel, transducer cannot image samples that are substantially concave or convex. Although rigid transducers with convex shapes have been used to image and measure concave surfaces, the complexity and costs associated with the provision of such a transducer are excessive. Further, when such shapes are used, they limit later movement of the transducer with respect to the sample. In using a rigid plano-parallel transducer with a convex surface sample, only small areas of the convex sample around the point of contact with the transducer can be imaged.
Fourth, in view of the problems associated with the possible presence of particulate matter on the sample, the danger of the transducer itself inflicting damage on the sample, and the difficulties associated with the transducer and sample having other than opposed parallel planar surfaces, achieving the required proximity between the transducer and the sample requires a high degree of operator skill.
Therefore, it is an object of the invention to provide an improved transducer for a photon tunneling microscope apparatus, the transducer being adapted for use in imaging and measuring sample surfaces having particulate matter thereon.
Another object of the invention is to provide such a transducer which is adapted to engage the sample surface without damaging the sample surface.
A further object of the invention is to provide such a transducer adapted to image and measure concave and convex sample surfaces.
A still further object of the invention is to provide such a transducer requiring less operator skill in achieving appropriate proximity between the transducer and the sample.
Other objects of the invention will be obvious, in part, and, in part, will become apparent when reading the detailed description to follow.